The objective of the proposed research is to study mechanisms by which the permeability of the muscle postjunctional membrane is increased and then blocked (receptor desensitization) during application of selected quaternary ammonium compounds. Principal attention is devoted to the role of calcium in mechanisms of receptor activation and desensitization. The work involves use of frog skeletal muscle fibers and application of various electrophysiological techniques such as intracellular recording of membrane potentials, point voltage clamping, multibarrel iontophoresis, etc.